160 BASTIN AND HILL! ORE DEPOSITS OF GILPIN COUNTY 



GEOLOGY. — Some features of the ore deposits of Gilpin County, 

 Colorado.^ Edson S. Bastin and J. M. Hill, U. S. Geo- 

 logical Survey. . 



Gilpin County and the adjacent portions of Boulder and Clear 

 Creek counties constitute one of the oldest mining districts in 

 Colorado. Central City, the principal mining camp of the 

 region, was the site of the first discovery of lode gold in Colo- 

 rado, made by John Gregory in the spring of 1859. The period 

 of greatest prosperity in this region was the seventies and 

 eighties, but it has remained an important producer of gold and 

 silver down to the present time, and will doubtless continue 

 so for many years to come. The region lies in the Front Range 

 of the Rocky Mountains, 30 to 40 miles west of Denver. The 

 principal mining centers are Central City, Idaho Springs, Neder- 

 land, Alice, and Dumont. 



The geologic groundmass of the region consists of pre-Cam- 

 brian rocks, both sedimentary and igneous in origin, and in part 

 dynamo-metamorphosed. The pre-Cambrian sediments are 

 mostly argillaceous and the igneous rocks granitic. Into this 

 groundmass Tertiary igneous rocks have been intruded as stocks 

 and dikes. Monzonite is the predominant rock type among these 

 intrusives, but many other rocks have been derived by mag- 

 matic differentiation from the monzonite magmas, titaniferous 

 iron ores and porphyry dikes representing the extreme products. 

 The Tertiary intrusives which are the ''porphyries" of the miners 

 are (with few exceptions) older than the ores. 



The ore deposits may be classified according to the metals which 

 give them their predominant value as follows: 



(1) Gold-silver ores. This class is the most widespread and 

 most productive and the only one that will be described in de- 

 tail in this paper. 



(2) Uranium ores. These are confined to a small area on 

 Quartz Hill near Central City, and are beHeved to be a phase of 

 the gold-silver ores. 



(3) Tungsten ores. These were not studied in detail by the 



' Published with the permission of the Director of the U. S. Geological Survey. 



